The present invention relates to a continuous process for the preparation of terephthalic aldehyde, i.e. the dialdehyde, through hydrolysis of .alpha.,.alpha.,.alpha.',.alpha.'-tetrachloro-p-xylene in the presence of catalysts.
Through the side-chain halogenation of p-xylene with radically excited chlorine or bromine, .alpha.,.alpha.,.alpha.',.alpha.'-tetrahalogen-p-xylene can be obtained, which is easily hydrolyzed at elevated temperature in the presence of catalysts by means of sulfuric acid according to the following sources: Belgian Pat. 667,022; M. Hoenig, Monatshefte 9, 1153; J. Thiele, O. Guenther, A. 347, 106-111 (1906); A. Weissberger, H. Bach, Ber. 65 (1932) 24, 28; Ruzicka, Buijs, Stoll, Helv. 15 (1932) 1220, 1222; Deluchat, A. (11) 1 (1934) 181, 207, 209; J. M. Snell, A. Weissberger, Org. Synth. Col., Vol. III (1955) 788, or by means of water according to A. Colson, H. Gautier, Bull. Soc. Chim. France (2) 45, 508 (1886); A. (6) 11, 28 (1887), G. Arditti, L. Palfray, C.r. 223., 548 (1946), and French Patent No. 2,059,339, either a great excess of water being used, from which the aldehyde can be obtained after the hydrolysis, or the hydrolysis being performed with the stoichiometric amount of water in the presence of preferably 0.5 to 30 m moles of iron salts per mole of dichloromethyl group, in a temperature range of 40.degree. to 140.degree. C., in accordance with French Pat. No. 2,059,339 (1970), the water being supplied in liquid form and the product being obtained in liquid form.